fix: isolate deadline reminder dispatch (Closes #711)

tests/test_deadline_reminders.py

@@ -1,3 +1,4 @@
+import asyncio
 import json
 from datetime import datetime, timezone
 
@@ -160,3 +161,43 @@ async def test_competing_workers_send_one_deadline_digest(tmp_path):
     assert competing == 0
     assert await active == 1
     assert len(sent) == 1
+
+
+@pytest.mark.anyio
+async def test_deadline_reminders_bound_fanout_and_isolate_failed_devices(tmp_path):
+    store = PushSubscriptionStore(tmp_path / "push.sqlite3")
+    for index in range(4):
+        session_id = f"device-{index}"
+        store.upsert(session_id, {
+            "endpoint": f"https://push.example/{session_id}",
+            "keys": {"p256dh": "public-key", "auth": "auth-secret"},
+        })
+        store.set_deadline_preferences(
+            session_id, enabled=True, timezone="UTC", reminder_hour=9
+        )
+    active = 0
+    peak = 0
+
+    async def assigned():
+        return {"complete": True, "items": [{"id": 1, "due_date": "2026-08-14T00:00:00Z"}]}
+
+    async def send(subscription, _payload):
+        nonlocal active, peak
+        active += 1
+        peak = max(peak, active)
+        await asyncio.sleep(0.01)
+        active -= 1
+        if subscription["endpoint"].endswith("device-1"):
+            raise RuntimeError("provider failed")
+
+    delivered = await dispatch_deadline_reminders(
+        store,
+        PushConfiguration("public", "private", "mailto:ops@example.com"),
+        assigned,
+        send,
+        now=datetime(2026, 8, 13, 10, 0, tzinfo=timezone.utc),
+        max_concurrency=2,
+    )
+
+    assert delivered == 3
+    assert peak == 2

tests/test_push_notifications.py

@@ -230,6 +230,51 @@ async def test_push_poll_authorizes_delivery_against_managed_session(monkeypatch
     assert captured["max_individual_notifications"] == 4
 
 
+@pytest.mark.anyio
+async def test_push_poll_still_dispatches_deadlines_when_unread_dispatch_fails(monkeypatch):
+    calls = []
+
+    sleeps = 0
+
+    async def no_wait(_seconds):
+        nonlocal sleeps
+        sleeps += 1
+        if sleeps > 1:
+            raise asyncio.CancelledError
+
+    async def fail_unread(*_args, **_kwargs):
+        calls.append("unread")
+        raise RuntimeError("unread unavailable")
+
+    async def dispatch_deadlines(*_args, **_kwargs):
+        calls.append("deadline")
+
+    monkeypatch.setattr(main.asyncio, "sleep", no_wait)
+    monkeypatch.setattr(main, "dispatch_unread_updates", fail_unread)
+    monkeypatch.setattr(main, "dispatch_deadline_reminders", dispatch_deadlines)
+
+    with pytest.raises(asyncio.CancelledError):
+        await main._push_poll_loop()
+
+    assert calls == ["unread", "deadline"]
+
+
+def test_dispatch_leases_are_isolated_by_channel(tmp_path):
+    path = tmp_path / "push.sqlite3"
+    first = PushSubscriptionStore(path)
+    second = PushSubscriptionStore(path)
+
+    assert first.acquire_dispatch_lease(
+        "unread-owner", channel="unread", now=100, lease_seconds=30
+    ) is True
+    assert second.acquire_dispatch_lease(
+        "deadline-owner", channel="deadline", now=100, lease_seconds=30
+    ) is True
+    assert second.acquire_dispatch_lease(
+        "other-unread-owner", channel="unread", now=100, lease_seconds=30
+    ) is False
+
+
 def test_replacing_subscription_resets_delivery_cursor_and_revocation_removes_device(tmp_path):
     store = PushSubscriptionStore(tmp_path / "push.sqlite3")
     original = {